The present invention relates to a charger using one or more solar batteries and useful for small-size apparatus such as an electronic wristwatch.
It is well known that a nickel-cadmium (Ni-Cd) battery can be used as a power source for electronic wristwatches in combination with one or more solar batteries (SB). However, the Ni-Cd battery is not preferable because cubic efficiency of battery capacity is poor and the amount of self discharging is large. For instance, when the Ni-Cd battery has a diameter of 11.6 mm and a height of 5.1 mm, the battery capacity is about 20 mAH and the self discharging amounts to nearly 30% for 90 days. In contrast, a sealed type silver oxide battery, which consists of a positive electrode made of Ag, AgO or Ag.sub.2 O and a negative electrode made of Zn or Cd, has excellent properties with respect to cubic efficiency, the self discharging, etc. For instance, in the case of a silver oxide battery of substantially the same size as the above, that is, a diameter of 11.56 mm and a height of 5.33 mm, the battery capacity is about 190 mAH and the amount of self discharging is about 10% per year when the electrolyte is NaOH and about 20% per year when the same is KOH. Moreover, the silver oxide battery shows charging efficiency up to about 90% while the Ni-Cd battery has a charging efficiency of only about 70%. Nonetheless, in the event that the silver oxide battery maintains its charge after full charging, the battery will swell out and the output voltage of the battery will swing. The former is deemed to occur due to the fact that gas is generated within the battery by the over-charging. The latter is deemed to occur because Ag oxide is dissolved into an alkali electrolyte and then shifted into the Zn negative electrode or Zn is deposited in a dendrite or spongy fashion to cause short circuiting.
Even in the prior art Ni-Cd battery, the battery might swell out by gas accumulation within the battery when more than 0.1 CA charging current flows under the full charged condition. To this end, a current limiting resistor or the like is usually connected to suppress the charging current in the order of less than 0.1 CA. However, unlike the Ni-Cd battery, the above-mentioned silver oxide battery will result in expansion of the battery or swinging of the output voltage even when the charging current is limited. A sealed type mercury battery consisting of a positive electrode made of Hg, HgO or Hg.sub.2 O and a negative electrode made of Zn or Cd, must undergo the same circumstances.
Therefore, it is an object of the present invention to provide a charger which overcomes the above-mentioned disadvantages. According to the present invention, a charging current limiting means and a charging voltage limiting means are provided simultaneously in a charger unit including one or more solar batteries and a secondary battery such as a silver oxide battery. In one preferred form, the charging current limiting means and the charging voltage limiting means are enabled only when the solar batteries deliver the output voltage.